LED lamps have advantages of relatively low power consumption and a relatively long lifetime compared to fluorescent lamps, incandescent lamps, three-wave fluorescent lamps, and similar light illumination devices. Many LED illumination devices are Direct Current (DC) devices. Since power supplied to consumers is normally Alternating Current (AC), to drive LED illumination devices, a converter may be necessary to convert AC into DC.
When LEDs are driven with alternating-current (AC) power (converted to DC), the LEDs may be classified as an isolation type or as a non-isolation type. For an isolation type LED device, there may be nominal risk of an electric shock because an LED load ground and an AC power supply ground are separated from each other. However, the manufacturing costs for an insulation type LED device may be relatively high.
FIG. 1 illustrates an isolation-type LED lamp driver circuit based on a flyback converter, in accordance with the related art. A flyback converter may be used because it requires only one high-voltage switching element and therefore the flyback converter has a simple structure that may be implemented at a relatively low cost.
AC power is full-wave rectified through a rectifier. A full-wave rectified signal may be converted to DC through a filter and then used. In some instances (as shown), a separated circuit may be needed to improve a power factor.
A flyback converter structure is used to transmit energy to the LED load insulated from the rectified power. In the flyback converter structure, energy is stored in a magnetizing inductance on the primary side of a transformer while a switch is turned on, and energy in the magnetizing inductance is transmitted to an LED load on the secondary side of the transformer when the switch is turned off.
A controller serves to control the on/off operation of the switch such that the secondary-side LED current has a desired value. In general, in order to control the secondary-side current, it is necessary to detect the secondary-side current and to feed back the detected secondary-side current to the controller. Since the primary side and the secondary side should be insulated from each other, it may be necessary to use an element (e.g. an opto-coupler), which transmits a signal through light that is used to give feedback to drive the current. In FIG. 1, a snubber serves to suppress high-voltage spark due to resonance caused by parasitic inductance when the switch is turned off. With this structure illustrated in FIG. 1, it is possible to control the LED current constant regardless of a fluctuation in the LED load, a fluctuation in the power supply voltage, or the like.
In some configurations, a flyback converter may have dimmer control leakage pull down using the flyback converter's main power device. The flyback converter's main power device may have a configuration in which a secondary-side current is predicted from a switching duty signal and a peak current transmitted from the primary side to the secondary side, and the peak of the secondary-side current is compared with a power supply voltage to be supplied, thereby controlling an LED current regardless of a fluctuation in an LED load or a fluctuation in power.
However, in an isolation-type LED lamp driver circuit using a flyback converter of the related art, it is necessary to provide an additional secondary-side current sensor circuit and an insulation element (e.g. an opto-coupler). Accordingly, isolation-type LED devices may have a relatively large system volume and/or relatively high manufacturing costs. From the viewpoint of cost reduction, it may be desirable to control the secondary-side current with no additional circuit in the isolated flyback converter. According, there may be a demand for a technique for controlling a current with no secondary-side additional circuit and with the function of shaping the secondary-side current in tune with an input voltage to minimize a phase difference and to improve a power factor. There may also be a demand for stably controlling a secondary-side current through only primary-side control, regardless of a fluctuation in the secondary-side load.